Autosomal dominant polycystic kidney disease (ADPKD), caused by mutations in PKD1 and PKD2 genes that encode polycystin-1 and -2 (PC1 and PC2), respectively, is the most common lethal monogenic genetic diseases (ADPKD) of man, affecting ~1 in 1,000 individuals. ADPKD leads to cystic replacement of renal tissue and progressive renal failure, requiring renal replacement therapy in half of the cases by age 50. It is a systemic disease involving the kidney, liver, pancreas, arteries and heart. Using cells derived from mice with a targeted Pkd1mutation, we have recently shown that cells lacking functional PC1 formed cilia but did not increase calcium influx in response to physiological fluid flow. Blocking antibodies directed against PC2 similarly abolished the flow response in wild-type cells as did inhibitors of the ryanodine receptor, whereas inhibitors of G-proteins, phospholipase C and IPS receptors had no effect. These data led us to propose a model in which PC1 and PG2 form a mechanosensitive receptor channel complex and mediate fluid flow sensation in primary cilia of renal epithelia. Two lines of investigations are proposed in this competitive renewal. First, we aim to validate our hypothesis in both immortalized and freshly isolated primary cells from normal human individuals and cysts of ADPKD patients. We will also determine whether haplo-insufficiency is sufficient to affect flow sensing ability of renal epithelia. Second, we propose to test the hypothesis that PC1 mediated G protein signaling pathway is modulated by flow stimulation. In the last funding period, we showed that PC1 acts as a G-protein coupled receptor and its activation of G protein is inhibited by PC2.Here we hypothesize that this inhibition is regulated by physiological stimulations such as urine flow and will further dissect the downstreampathways of PC1 mediated G protein signaling. We believe our studies are of high significance and will provide novel insights into polycystin function and the molecular mechanisms underlying the pathogenesis of polycystic kidney disease. These studies are also likely to facilitate the development of therapeutic strategies.